This application is related to the patent application entitled: xe2x80x9cMethods And Apparatus For Providing Short RACH Frames For Fast Latency,xe2x80x9d filed concurrently herewith.
The present invention relates to methods and apparatus for providing power ramping in a communications system and, more particularly, to methods and apparatus for providing enhanced power ramping via multi-threshold detection in a receiver of a Universal Mobile Telecommunications System.
A major effort has been underway in the last decade to integrate multimedia capabilities into mobile communications. The International Telecommunications Union (ITU) and other organizations have been attempting to develop standards and recommendations that ensure that mobile communications of the future will be able to support multimedia applications with at least the same quality as existing fixed networks. Particularly, many global research projects have been sponsored in order to develop such next (third) generation mobile systems. Research and Development of Advanced Communication Technologies in Europe, RACE-1, and RACE-2, and Advanced Communications Technology and Services (ACTS) are examples of such efforts in Europe. It is known that in order to provide end users with the requisite service quality for multimedia communications, Internet access, video/picture transfer, high bit rate capabilities are required. Given such requirements, bearer capability targets for a third generation system have been defined as 384 kilobits per second (kb/s) for full coverage area and 2 Megabits per second (Mb/s) for local area coverage.
Universal Mobile Telecommunications System (UMTS) is a new radio access network based on 5 Megahertz Wideband Code Division Multiple Access (W-CDMA) and optimized for support of third generation services including multimedia-capable mobile communications. Since major design goals of UMTS are to provide a broadband multimedia communications system that integrates infrastructure for mobile and fixed communications and to offer, inter alia, the same range of services as provided by the fixed and wireless communications networks, UMTS must provide circuit-switched as well as packet-switched services, a variety of mixed media traffic types, and bandwidth-on-demand. However, providing multimedia support implies the need for flexibility, that is, being able to support services with different bit rates and Eb/N0 requirements, and to multiplex such services in a multiservice environment. UMTS is designed to be able to support such demands.
Referring to FIG. 1, an exemplary block diagram of a UMTS access network is shown. Particularly, a plurality of remote terminals 2 and 4 (e.g., mobile terminals) communicate with base stations (NODE-B) 6 via W-CDMA wireless links 8. The remote terminals may be a variety of devices such as a wireless phone 2 or a portable personal computer 4 with an internal or external modem. In the UMTS standard, a base station is called a NODE-B. These base stations communicate with a network component that provides radio resource management functions and is called a Radio Network Controller (RNC). Since UMTS is a W-CDMA system, soft handoffs are supported. In the case of soft handoffs, there are two base stations 6 serving one remote terminal. Thus, the remote terminal sends frames to these two base stations. When the two base stations receive the frames from the remote terminal, they send them to a Frame Selector Unit (FSU). The FSU decides which is a better frame, in terms of frame quality, to be sent to the core network. In UMTS, the FSU may be physically integrated with the RNC and as such, in FIG. 1, the RNC and FSU are shown as block 10, but also are separated functionally as block 12 (FSU) and block 14 (RNC). Other elements in the UMTS network perform conventional functions such as: the xLR databases 20, which provide home and visiting location information; and the interworking function (IWF) units. It is to be appreciated that the Universal Mobile Switching Center (UMSC) 16 serves as the mobile switching center for the base stations 6 in the UMTS. Sub-networks 18 are wireless service provider networks and CNI through CNn are the core networks 24 to which the remote terminals are ultimately coupled.
Referring to FIG. 2, a diagram of the typical protocol stack in UMTS is shown. In UMTS, Layer 1 (L1) is the physical layer (PHY) which offers information transfer services to the MAC (Media Access Control) layer and higher layers. The physical layer transport services are described by how and with what characteristics data is transferred over the transport channels of the radio interface. Layer 2 (L2) is comprised of sublayers which include MAC, LAC (Link Access Control), and RLC and RLCxe2x80x2 (Radio Link Control). In UMTS, the functions performed in RLC are split and thus two RLC protocols (RLC and RLCxe2x80x2) are specified. The RLC and MAC layers provide real-time and non-real-time services. The MAC layer controls but does not carry out the multiplexing of data streams originating from different services. That is, the MAC layer, via logical channels, allows common physical communications channels (e.g., broadcast channel) to be shared by a number of remote terminals. IP (Internet Protocol) is the network layer.
xe2x80x9cUuxe2x80x9d refers to the UMTS-specific interface between a remote terminal and a base station, while xe2x80x9clubxe2x80x9d refers to the UMTS-specific interface between a base station and the RNC/FSU. Layer 2 of the radio access network (i.e., left side of NODE-B on the protocol stack) is split into RLC and MAC layers, while Layer 2 of the core network (i.e., right side of NODE-B on the protocol stack) is more related to the technology used to transport network layer frames, e.g., ATM (Asynchronous Transfer Mode) or Frame Relay. IP is shown as the transport protocol, however, UMTS is not so limited. That is, UMTS can cater to other transport protocols. Further details on the protocol layers may be found in Dahlman et al., xe2x80x9cUMTS/IMT-2000 Based on Wideband CDMA,xe2x80x9d IEEE Communications Magazine, pp. 70-80 (September 1998) and in ETSI SMG2/UMTS L2 and L3 Expert Group, xe2x80x9cMS-UTRAN Radio Interface Protocol Architecture; Stage 2,xe2x80x9d Tdoc SMG2 UMTS-L23 172/98 (September 1998).
One of the logical channels associated with the media access control (MAC) protocol of UTMS is the random access channel (RACH). RACH is an up-link common transport channel used to carry control information and short user packets from a remote terminal. Referring to FIG. 3A, a block diagram of an exemplary hardware implementation of a non-coherent RACH detection algorithm for use in a UMTS base station (NODE-B in FIG. 1) is shown. The RACH receiver 30 is capable of providing the following functions: detection, demodulation and decoding, and acknowledgement. The purpose of detection is to determine if a RACH burst (i.e., access request signal) is being sent by a remote terminal and to resolve the strongest multipath components of the incoming burst. The receiver 30 also demodulates and decodes the message contained within the corresponding RACH to ascertain the remote terminal identifier and the requested service. After decoding a remote terminal RACH transmission, the receiver generates an acknowledgement signal which the base station transmits to the remote terminal over a Forward Access Channel (FACH).
The RACH receiver 30 preferably performs the above functions in accordance with the following structure. A RACH transmission burst is received and demodulated by mixers 32 and then filtered in filters 34. The signal is then sampled in sampling unit 36. Despreader 38 decodes the signal in accordance with the spreading sequence, in this case, 512 Gold code. The decoded signal is buffered (buffer 40) and sent to time shifting unit 50. Also, the output of the despreader 38 is provided to integrator 42. The outputs of the integrator 42 are mixed (mixer 44) and provided to timing detector 46 and then threshold detector 48. The output of the threshold detector 48 indicates whether a valid signal was received from the remote terminal. This result is provided to time shifting unit 50. If it is a valid signal (e.g., above pre-determined threshold), the decoded signal is then down-sampled by unit 52. Then, depending on the preamble, described below, the signal passes. through the 16 tap filter unit 54 to the preamble signature searcher 56. The output of the searcher 56 provides the base station with the encoded remote terminal""s identifier and information as to the service(s) requested by the remote terminal. The encoded information is then decoded by a convolutional decoder 58 and checked by a CRC (cyclical redundancy check) decoder 59.
Referring to FIG. 3C, a block diagram of an exemplary hardware implementation of an uplink transmitter 60 for use in a UMTS remote terminal (e.g., remote terminals 2 and 4) is shown. In a UMTS remote terminal, data modulation is dual channel QPSK (quaternary phase shift keying), that is, the I and Q channels are used as two independent BPSK (binary phase shift keying) channels. For the case of a single uplink DPDCH (dedicated physical data channel), the DPDCH and the DPCCH (dedicated physical control channel) are respectively spread by two different channelization codes (CC and CD) via mixers 62 and 64 and transmitted on the I and Q branches. The I and Q branches are multiplexed in IQ MUX 66. The total spread signal I+jQ is then complex scrambled by a connection-specific complex scrambling code in mixer 68. The real portion of the signal is then filtered in root-raised cosine filter 70, while the imaginary portion of the signal is filtered in root-raised cosine filter 72. The output of filter 70 is modulated in mixer 74 with a cos (xcfx89t) signal. The output of filter 72 is modulated in mixer 76 with a xe2x88x92sin (xcfx89t) signal. The two modulated signals are then added in adder 78. The composite signal is then amplified to a predetermined signal strength (i.e., power level) in amplifier 80 and then transmitted by an antenna (not shown). A similar arrangement may be used in the base station.
Referring back to FIG. 3B, a graphical representation illustrating how the detection algorithm works in the existing UMTS receiver is shown. When the signal strength of an access request signal sent by a remote terminal to a UMTS base station exceeds a detection threshold, e.g., DTHRESH1 (equal to 7 dB), the receiver can detect the signal and pass the message on to the convolutional decoder 58 and the CRC decoder 59. If the CRC is correct, then the receiver sends an acknowledgement signal to be transmitted to the sender. This may be accomplished through the transmission section of the base station, as is known. That is, the transmission section receives the indication from the CRC decoder 59 of the receiver and, in response, generates and transmits the acknowledgement signal. However, the receiver cannot differentiate signals falling below that single threshold. The problem with this deficiency is that the receiver does not distinguish between valid access signals that merely have a weak signal strength versus noise or collision-effected signals. For example as shown in FIG. 3B, while the receiver can detect Signal 1 sent from a remote terminal, it does not detect a signal having a signal strength falling below the detection threshold level, i.e., Signal 2, since the existing UMTS receiver has only a single detection threshold. With the existing detection algorithm, the receiver only sends an acknowledgement signal (e.g., xe2x80x9ccorrect receptionxe2x80x9d signal) to the sender if the signal exceeds DTHRESH1 and the CRC of the access request message is correct. For all other cases, the sender needs to increase the signal strength by, for example, 3 dB (e.g., by adjusting output amplifier 80). However, the power increase of 3 dB may be too much if the original signal power was just below DTHRESH1. This may then result in saturation of the receiver or cause interference with other signals being transmitted in the area.
The present invention provides methods and apparatus for providing multi-detection thresholds in a receiver of a communications system such as, for example, the UMTS. In this manner, a receiver implementing the invention can differentiate signals falling below a single threshold. Advantageously, the receiver distinguishes between valid access signals that merely have a weak signal strength versus noise or collision-effected signals. Also, the invention allows the sender to increase its transmit power level incrementally based on the content of a message received from the receiver, thereby reducing the possibility that the sender will increase signal strength resulting in saturation of the receiver or interference with other signals being transmitted in the area.
In one aspect of the invention, a method for use in a receiver of detecting a signal transmitted by a transmitter comprises determining whether the signal is greater than or equal to at least a first power threshold value when the signal is below an initial detection threshold value, and informing the transmitter when the signal is greater than or equal to the first power threshold value such that the transmitter can increase a signal strength of the signal by a first predetermined amount and re-transmit. Preferably, the method also includes determining whether the signal is greater than or equal to a second power threshold value when the signal is below the initial detection threshold value and the first power threshold value, and informing the transmitter when the signal is greater than or equal to the second power threshold value such that the transmitter can increase the signal strength of the signal by a second predetermined amount and re-transmit. Still further, the method preferably includes providing the transmitter with no indication when the signal is not greater than or equal to the second. power threshold value such that the transmitter can increase the signal strength of the signal by a third predetermined amount and re-transmit. Also, the method preferably includes informing the transmitter when the signal is greater than or equal to the initial detection threshold value but included an invalid CRC code such that the transmitter can re-transmit.
In another aspect of the invention, a method for use in a transmitter of power ramping a signal transmitted by the transmitter and received by a receiver comprises increasing a signal strength of the signal by a first predetermined amount when informed by the receiver that the signal is greater than or equal to a first power threshold value but below an initial detection threshold value, and re-transmitting the signal. The method also preferably includes increasing the signal strength of the signal by a second predetermined amount for re-transmission when informed by the receiver that the signal is greater than or equal to a second power threshold value but below the initial detection threshold value and the first power threshold value. Still further, the method preferably includes increasing the signal strength of the signal by a third predetermined amount for re-transmission when no indication is received from the receiver. Also, the method preferably includes re-transmitting the signal when informed by the receiver that the signal is greater than or equal to the initial detection threshold value but included an invalid CRC code.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.